Recent EPA mandates are now placing more stringent controls on emissions from various sources, including diesel engines and small 2- and 4-cycle engines such as lawnmower, weedeater, chainsaw, and motorcycle engines.
Because of its low manufacturing costs and simplified operating details, the two-cycle engine is favored for small, low-cost engine applications. However, because the exhaust ports remain open for a time after the intake ports are blocked, and due to mixing of the clearance and combustion gases, a finite amount of combustible gases are vented without undergoing any reaction. Analysis of the exhaust indicates that the majority of hydrocarbon emissions are due to direct fuel loss. The bulk of CO emissions arise from incomplete burning of the fuel charge, due to rich operating conditions and low volumetric efficiency. Similar exhaust characteristics occur in side-valve 4-stroke engines.
Because of their operating conditions and minimal maintenance, aftertreatment and engine modifications for two-cycle and side-valve engines must take into account such factors as large load and speed range requirements, heavy vibration, frequent on-off operations, and multipositional operating characteristics. In addition, due to the fuel/oil, mixture ratio required for operation of two-stroke and heavy hydrocarbon emissions in low-cost 4-strokes, any modifications must be insensitive to misfueling, backfiring through the exhaust port, and contaminants in the oil (such as mercaptans containing sulfur, etc.). Also, any engine modifications/converter control options must require minimum or zero maintenance and not introduce any potential safety problems (such as excessive surface or exhaust temperature).
Numerous methods have been utilized in the past to reduce emissions. These emissions control technologies generally fall into two categories: engine modifications and exhaust aftertreatment. The first method utilizes modifications of the engine cycle to achieve more efficient and complete combustion of the fuel and is primarily focused on control of the combustion process. Engine modifications include control of the air/fuel mixture ratio, air/fuel/clearance gas mixing, and combustion process/timing to achieve a more efficient, complete, and uniform combustion process. Exhaust aftertreatment consists of transforming the remaining pollutants into less harmful compounds and can be catalytic or thermal.
A catalyzed exhaust aftertreatment system is the lowest in cost and performance impact and is the most easily implemented emission control option for small two-stroke (and four-stroke) utility engines. Studies have shown this type of system to be capable of reducing emissions to extremely low levels. Aftertreatment utilizes a catalytic surface to enhance the combustion of hydrogen, hydrocarbons, and CO. With the design of the present invention, the catalyst system also traps and combusts organic particulate matter and can be used to raise exhaust gas temperature to allow thermal reactions to complete pollutant conversion.
The major problems in catalytic converters for two-cycle engines arise primarily from the extreme levels of unreacted fuel and CO in two-cycle engine exhaust, which lead to excessive heat release. This heat release, combined with the close proximity to the engine cylinder creates excessive temperatures, which in turn cause degradation of the catalyst/support as well as presenting fire and burn hazards. Other less obvious but significant problems arise from the high-temperature poisoning effect on the catalyst/catalyst support system, the reduced residence times available (and therefore higher conversion kinetics required) in the small engine vs automotive engine, the presence of sulfur derived from the fuel and lubricating oils, and the substantially increased vibration and impact load durability required in the utility engine as compared to automotive applications.
Typical current generation automotive catalysts consist of a three-way catalyst (composed of a gamma-alumina washcoat impregnated with Pt-Rh catalysts) supported on a through-flow cordierite honeycomb substrate. This cordierite support is not adaptable to the small utility engine due to temperature and reaction kinetic concerns. Another substrate material that has been demonstrated is the use of wire meshes, screens, or expanded metal. However, these systems are too expensive for adaptation to small utility engines since complete converter systems can cost approximately $70-150.
The converter must carry out the following three chemical reactions: EQU CO+1/20.sub.2 .fwdarw.CO.sub.2 EQU C.sub.n H.sub.(2n+2+)1/2(3n+1)O.sub.2 .fwdarw.nCO.sub.2 +(n+1)H.sub.2 O EQU CO+NO.fwdarw.N.sub.2 +CO.sub.2
Of these reactions, the reduction of NO to nitrogen is by far the most demanding. However, NO.sub.x reduction is not required to meet 1999 two-cycle emissions standards unless significant engine changes result in an increase in NO.sub.x emissions. The modern autocatalyst is supported on a cordierite honeycomb (cost of $7-14/unit) and comprises a suitable mixture of platinum, rhodium, and palladium metals supported on a thin, high-area alumina "washcoat". A typical automotive catalyst would utilize a 300 cell/in.sup.2 cordierite monolith with 0.83 g of platinum applied as a ceria/alumina/platinum slurry having a density of 110 g of slurry/g of platinum. Some 60 .mu.m of slurry is applied to the cordierite followed by drying and calcining at 500.degree. C. Approximately 3-5 grams of Pt-group metals are required for each converter.
The two primary difficulties with (The use of an Automotive) a catalyst for two-cycle engines lies in the unsuitability of the washcoat and cordierite substrate to survive the excessive temperatures and thermal transients generated during oxidation of the high-energy exhaust mixture, so the low residence times obtained in such a through-flow geometry and the heavy vibrations encountered in a one-cylinder engine. At full throttle, complete conversion of the small engine exhaust products results in temperatures exceeding 1000.degree. C. At high temperatures, three phenomena take place which "poison" the current catalyst/support materials.
First, at .apprxeq.900.degree. C., the gamma-alumina used as a high surface area washcoat transforms into alpha-alumina, having less than 1/300 of the surface area. This condition can be alleviated by starting with an alpha-alumina washcoat, but this washcoat does not provide nearly the surface area of gamma-alumina.
Second, platinum, rhodium, and palladium migrate into the alumina lattice and become ineffective as the lattice is not exposed to the exhaust gases, and this effect is compounded by sintering in both the alumina and metallic phases.
Third, at elevated temperatures mobile species from the support (usually SiO.sub.2) can diffuse over the surface of the metal or oxide catalyst, reducing the exposed surface area and poisoning the active surface sites. In addition to catalyst poisoning, cordierite becomes extremely soft at .apprxeq.1000.degree. C. and loses most of its mechanical properties, and due to the nature of a small engine develops "hot spots" and localized melting of the support. Substantial grain growth also occurs which leads to thermal "ratcheting" and eventual failure under cyclic operation.
In addition to their temperature limitations, current cordierite honeycomb supports are not available to cell sizes and geometries suitable for two-cycle exhaust systems. To achieve the desired residence times and surface activity in small muffler cans, finer cell sizes and higher strength supports will be required, in an alternate higher thermal conductivity material.